This invention relates to gas discharge lasers and in particular short pulse gas discharge ultraviolet lasers such as KrF, ArF and F2 lasers.
Kryton fluoride gas discharge lasers producing 248 nm ultraviolet light are currently widely used as light sources in the integrated circuit lithography industry. The lasers typically operate in burst modes at duty factors of 20-40 percent around the clock 7 days per week. These lasers permit integrated circuit fabricators to produce integrated circuits with critical dimensions of about xc2xc micron. In the future, the industry is expected to shift to light sources at smaller wavelengths in order to support even smaller critical dimensions. These sources for the most part are expected to be ArF lasers which produce a pulse beam at about 193 nm and F2 lasers which produce a pulse beam at about 157 nm. These lasers typically operate at pulse repetition rates of about 1000 to 2500 pulses per second with pulse durations of about 20 to 50 ns.
Refractive optical components (including large lenses) of lithography machines used in integrated circuit fabrication are very expensive, costing several million dollars per machine. It is known that high intensity ultraviolet light causes deterioration of these refractive components and that the rate of deterioration increases with decreasing wavelength and also with the laser intensity. One form of optical damage known as two photon absorption is non-linear, increasing with the square of laser power intensity. In order to increase production of integrated circuit fabrication lines, there is a need to increase the power of the light sources.
The above issues were discussed in detail in U.S. Pat. No. 6,067,311 (hereby incorporated by reference) assigned to Applicant""s employer. That patent discloses a pulse multiplier configurations for dividing each pulse from an excimer laser into multiple pulses such as four separate pulses each spaced apart in time by a time period approximately equal to the duration of the original pulse. This has the effect of stretching the pulse time-wise and reducing its instantaneous power without a substantial reduction in the energy of the pulse. One of the embodiments of that application is reproduced herein as prior art FIG. 1. In the FIG. 1 40 ns pulses of a ArF laser 50 are stretched to about 100 ns with a pulse multiplier unit. This is accomplished using two polarization beam splitters 58 and 62, six quarter wave plates and four maximum reflection mirrors. Legs of this system are 6 meters and 12 meters producing delays of 20 ns and 40 ns respectfully.
One problem with the system described in FIG. 1 as well as similar pulse stretchers is that laser beams from gas discharge lasers are usually slightly divergent and the amount of divergence changes over the life of the lasers. Therefore, if a pulse is separated into four parts with each of the parts following different path lengths before being recombined, the various portions of the pulses could have different beam size and angular spread. This is very undesirable.
What is needed is a better beam stretcher for gas discharge laser light sources.
The present invention provides a gas discharge laser with a pulse multiplier. In the pulse multiplier, short pulses from the laser are divided into portions and all or all but one of these portions are delayed in delay legs by different time periods and recombined to provide a stretched output pulse having substantially reduced intensity and longer duration as compared to the pulse from the laser. Focusing optics are included in each delay leg to assure that beam size and angular spread of each portion of the combined pulse is not substantially different from other portions of the pulse.